Control systems for electrostatic powder spraying apparatus

ABSTRACT

A method for controlling operation of powder spraying coating apparatus comprising automatically reducing both the discharge current and discharge voltage of the electrostatic charging means as the spray apparatus approaches a workpiece.

FIELD OF THE INVENTION

This invention concerns control systems for electrostatic powderspraying apparatus.

BACKGROUND OF THE INVENTION

Electrostatic spraying apparatus, especially for use in paintingworkpieces, generally comprise a duct for conveying gas-borne powder andmeans for electrostatically charging the powder, whereby it adheres tothe workpiece. Typically a corona discharge needle electrode is used tocharge the powder.

Corona charging of paint powder is not without its shortcomings althoughit is a preferred method of spray coating for a majority of powdercoaters. Problems associated with Faraday Cages, back ionisation orangepeel (pitting) and overcharging have been well documented but advantagesof consistency, good charge transfer and fast powder deposition largelyoutweigh the above-mentioned shortcomings.

The effects of such shortcomings may be reduced by spray nozzle designor by addition of earth robbing electrodes behind the spray nozzle, aswell as by careful setting of the spray apparatus position and operatingparameters.

Significant improvements can also be achieved by controlling thedischarge current instead of the discharge voltage which has proved itsworth over 25 years but surprisingly this method is still the exceptionrather than the rule. With this system the maximum discharge current(μA) is limited to a value determined by the operator and the dischargevoltage (kV) is allowed to float. The result is that as the sprayapparatus approaches the workpiece and the set discharge current isreached and stabilises, so the output voltage automatically reduces torelatively low levels, thus limiting the charge when the apparatus isclose to the workpiece. This is of great benefit in maintaining chargeconsistency, penetrating Faraday Cages and reducing back ionisation andorange peel appearance.

With conventional voltage control, where the discharge voltage is set bythe operator, the current rises exponentially as the spray apparatusapproaches the product which has the opposite effect to that of usingcurrent control and can also cause severe “over charging” of the powderand surrounding air and can give rise to potentially dangerous sparks,although safety cut out circuits are usually employed.

Despite the many advantages of current controlled corona charging, it isstill not perfect because the output voltage is allowed to float, whenthe spray apparatus is pulled away from the workpiece the current canfall below its “constant” or controlled setting and the voltage can riseto the maximum available from the generator, usually 80+kV. In somecases this may be too high and create strong field lines, with theresulting Faraday Cage difficulties. As the gun approaches the product,the air space between the gun and the product obviously reduces and willtherefore accept a progressively smaller charge from the corona needlein terms of free ions. It is the free ions which charge the airmolecules and then transfer the charge to the powder. The closer thespray apparatus is to the workpiece so the greater the proportion offree ions which are attracted to the surface of the workpiece as opposedto dissipating in the surrounding air, or being neutralised by metalwalls of the spray booth. This high charge on the surface of the powderis the main cause of the orange peel effect.

An aim of a first aspect of the present invention is to provide animproved method of controlling operation of powder spray coatingapparatus whereby the above-mentioned shortcomings of prior art methodsmay be overcome or at least reduced in effect.

In conventional electrostatic spraying apparatus, the flow of powderfrom a container to the means for electrostatically charging the powderis controlled by compressed air. As the spray apparatus approaches aworkpiece, the surface area which is sprayed reduces and hence thevolume of powder paint required also reduces. In practice, as the sprayapparatus approaches the workpiece, an operator will manually reduce thevolume of powder flowing to the electrostatic charging means by, forexample, reducing the pressure of the compressed air. However, since thevolume of powder sprayed is controlled manually, this can lead toirreproducible coating results.

An aim of a second aspect of the present invention is to provide anelectrostatic spraying apparatus which overcomes the afore-mentionedproblem.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a methodfor controlling operation of powder spray coating apparatus in order toimprove coating, comprising controlling the output power charging thepowder by progressively reducing both the discharge current anddischarge voltage as the spray apparatus approaches a workpiece whereinsaid reduction in the discharge current and discharge voltage commencesat a distance of separation between the spray apparatus and workpiecewhich is at least 100 mm.

Preferably, the powder spray coating apparatus comprises means forautomatically reducing the discharged current and discharge voltage asthe spray apparatus approaches the workpiece. In a preferred embodimentof the invention, the reduction in the discharge current and dischargevoltage is reversible as the spray apparatus moves away from theworkpiece. It is preferred that said means for automatically reducingthe discharge current and discharge voltage also, automatically,increases the discharge current and discharge voltage as the sprayapparatus moves away from the workpiece.

Preferably the discharge current can be set not to exceed an upper limittypically of 50 μA but possibly alternatively of, say, 30 μA. It ispreferred that, in use, said means for reducing the discharge currentand hence the output power comes into operation when the dischargecurrent reaches said pre-set upper limit, ie the threshold dischargecurrent. Similarly, said means for increasing the discharge current whenthe spray apparatus moves away from the workpiece may stop its operationwhen said discharge current reaches its pre-set upper limit.

Preferably, the maximum discharge voltage may be set by the apparatusoperator, which in turn also controls the normal operating level of thedischarge current.

In accordance with a further embodiment of the invention, the powderspray coating apparatus may further comprise means for reducing thevolume of powder dispensed by the spray apparatus as the spray apparatusapproaches the workpiece. Said reduction in the volume of paint may bein response to a decrease in the output power.

According to a second aspect of the present invention there is providedan electrostatic powder spray coating apparatus comprising anelectrostatic charging means and means for controlling the volume ofpaint sprayed in response to a change in discharge current or dischargevoltage of the electrostatic charging means.

As discussed above, with conventional electrostatic spray apparatuswhere the discharge current is controlled, as the spray approaches aworkpiece the discharge voltage reduces. This effect is also observedwhere the discharge voltage is controlled. Hence, the present inventionprovides a system whereby the volume of paint sprayed automaticallyreduces as the spray apparatus approaches the workpiece and thereforereproducible coating results can be obtained. Similarly, as the sprayapparatus is moved away from the workpiece, the discharge voltageincreases and therefore the volume of paint sprayed will also increasecorrespondingly.

It is preferred that said control means reduces the volume of paintsprayed in response, to a reduction in the discharge voltage and,conversely, increases the volume in response to an increase in thedischarge voltage. The control means is preferably dependent upon theactual value of the discharge voltage. Therefore, at predeterminedminimum and maximum values of discharge voltage, the control means mayallow, respectively, a minimum and maximum volume of paint to be sprayedPreferably, volume of paint flowing to the electrostatic charging meansis controlled such that it is proportional to the discharge voltage.

Compressed air is utilised to convey powder to the electrostaticcharging means and hence the flow rate of the paint is dependent uponthe pressure of the air and its flow rate. Hence, the volume of paintconveyed to the electrostatic charging means is also dependent uponthese two factors. The control means may control the air pressurethereby controlling the flow rate of the paint and, as a result, thevolume of paint flowing to the electrostatic charging means.Alternatively, the flow rate of the compressed air may be controlled byvarying the size of an orifice through which the air passes.

The volume of paint flowing to the electrostatic charging means may alsobe controlled by varying the size of an orifice through which the powdermay flow. The control means may control the size of said orifice.

Said control means may comprise a regulator which controls the pressureof the air pressure such that said pressure is proportional to thedischarge voltage. The control means may comprise a proportional controlvalve.

In an alternative embodiment, the delivery of powder from a container tothe compressed air means is controlled by said control means therebycontrolling the volume of powder conveyed to the electrostatic chargingmeans.

There are many known methods of delivering powder to said compressed airmeans, e.g. by use of a vibrational hopper feeder or a screw feederdevice. In the case of a vibrational hopper feeder, the frequency ofvibration may be controlled in order to vary the volume of powderdelivered. If a screw feeder device is utilised then the speed ofrotation of the screw may be controlled in order to vary the volume ofpowder delivered to the compressed air means in response to a change inthe discharge voltage.

Said delivery of the powder may, conceivably, be by way of a conveyorbelt and the speed of the belt may be controlled so as to vary thevolume of powder delivered.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention will now be further described, by way of example only,with reference to the accompanying drawings, in which:

FIGS. 1 to 3 are respectively graphs illustrating voltage control only,current control only and combined voltage and current control accordingto the invention;

FIG. 4 is a circuit diagram for providing combined voltage and currentcontrol for a powder spray coating gun of the invention; and

FIGS. 5 and 6 are simplified circuit diagrams of different aspects ofthe circuit shown in FIG. 4;

FIG. 7 is a block diagram of an embodiment of the present inventionaccording to a second aspect; and

FIG. 8 is a schematic of an internal charging gun according to a furtherembodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1 of the accompanying drawings, a graph is shown ofvoltage against distance and current against distance when the dischargevoltage only is controlled in operating of a powder spray gun. Thevoltage level decreases as the gun nears the workpiece due to thecharacteristics of the voltage regulator. Unfortunately with no controlover the current, this rises exponentially as the gun nears theworkpiece. This can result in overcharging of the powder and surroundingair and give rise to dangerous sparks.

FIG. 2 illustrates the prior art alternative which is to control thedischarge current. This has two problems that are shown by the graph,which again plots voltage against distance and current against distancebut for three different current levels namely 50, 30 and 10 μA. Thefirst problem is that as the gun is moved away from the workpiece andthe current falls below its controlled setting, the discharge voltagerises up to the maximum available from the high voltage generator. Thiscan be too high a voltage causing strong field lines and resulting inFaraday Cage difficulties.

The second problem is that as the gun approaches the workpiece the airspace between becomes smaller, so that more of the free air ions thatare produced are attracted to the workpiece and cause the “orange peel”effect on the workpiece surface.

The system according to the present invention of controlling both thedischarge voltage and discharge current is illustrated in FIG. 3. As canbe seen the discharge current has a maximum value set, eg. 50 μA or 30μA, but as the gun approaches the workpiece, the current is brought downto avoid the problem of excess air ions causing “orange peel” effect onthe workpiece surface. At the same time the discharge voltage is broughtdown which helps to overcome Faraday Cage problems.

When the gun is moved away from the workpiece the discharge currentrises to its threshold then falls off but the voltage may be controlledby the operator so as to remain below the maximum available from thehigh voltage generator.

FIG. 4 of the accompanying drawings shows a control circuit forachieving control of current and voltage as illustrated in FIG. 3.

With reference to FIGS. 4, 5 and 6, the circuit is indicated generallyby numeral 10 and comprises a 24 V d.c. supply 12, a reference voltageIC1, a dual operational amplifier comprising a first and second Op-ampIC2 a and IC2 b, a first transistor TR1, a second transistor TR2, apower resistor IRO, potentiometers VR1, VR2 and VR3, an oscillator 14,H.T. transformer 16 and an E.H.T. multiplier 18.

The voltage Vosc across oscillator 14 is dependent upon, the conductingstate of transistor TR2 which is itself dependent upon the voltageacross potentiometer VR3 for reasons which will become apparenthereinafter.

The input to the non-inverting terminal 3 of Op-amp IC2 a is set at areference voltage V ref which is generated by voltage reference IC1 andpotentiometer VR1. The input to the inverting terminal 2 of the Op-ampis set by potentiometer VR3.

The non-inverting terminal 5 of Op-amp IC2 b is held at a voltage whichis set by potentiometer VR2 and the reference voltage IC1. The input tothe inverting terminal 6 is dependent upon the voltage across resistorIRO.

Assuming that the 24 V supply voltage is reasonably constant and thatthe efficiency of the DC-DC conversion of the circuit is alsoapproximately constant then the output power P out of the EHT multiplier18 will be proportional to the input power as determined by the voltageacross the oscillator and the current flowing therethrough. The currentthrough resistor IRO is that through oscillator 14 and therefore thevoltage V IRO across resistor IRO is also proportional to P out. DiodeD1 only conducts when the voltage Vf across diodes D2 plus voltage Vbeof transistor TR1 is greater than the output voltage of IC2 b.

The operation of the circuit will now be described with, initially, thespray apparatus spaced from the workpiece at, say, 100 cm. If voltageVosc increases then the voltage at terminal 2 of IC2 a also increases.As a result, the output voltage of Op-amp IC2 a decreases and the basecurrent of TR1 is reduced thereby decreasing the current flowing throughTR1. This has the effect of reducing the base-emitter voltage VBE2 oftransistor TR2 which increases voltage VEC2 of the transistor and hencevoltage Vosc is decreased i.e. voltage Vosc falls to compensate for theinitial rise. If voltage Vosc falls then the opposite changes to theabove occur and hence voltage VEC of TR2 decreases and therefore voltageVosc rises. Hence, in the above described manner, the circuit as shownin FIG. 5 maintains the oscillator voltage Vosc at a constant value ie.the output power is held constant, when the spray apparatus is at agiven distance from the workpiece.

From the above it will be seen that the voltages at terminals 2 and 3 ofOp-amp IC2 a dictate the value of voltage Vosc and consequently theoutput power P out of EHT multiplier. Hence, potentiometers VR1 and VR3control the maximum output power P out.

As the spray apparatus approaches the workpiece, the output power P outof the EHT multiplier increases. As mentioned above, the voltage V IROacross resistor IRO is proportional to the output power and thereforethis voltage also increases with increasing P out. The increase ofvoltage at terminal 6 of Op-amp IC2 b results in a reduction in theoutput voltage of the Op-amp.

Consequently, as the spray apparatus continues to approach theworkpiece, the output voltage of Op-amp IC2 b decreases and a point isreached when this voltage is less than Vf+Vbe1 ie diode D1 begins toconduct. The output voltage of IC2 b is, of course, dependent upon thetwo inputs on terminals 5 and 6. Initially, potentiometer VR2 is variedsuch that the value of the output voltage of Op-amp IC2 b is less thanVf+Vbe1 when a predetermined output power or discharge power is reached.Therefore, the discharge current at which the output voltage of Op-ampIC2 b is less than said voltage Vf+Vbe1 ie the threshold current, can beset at any given value and may, for example, be set at 30 μA or 50 μA.In this manner the discharge current at which the circuit begins toreduce the discharge power P out can be pre-set.

When D1 conducts, a part of the current flowing into the base of TR1starts to flow through D1 and therefore TR1 conducts less. Hence, VEC ofTR2 increases and therefore Vosc decreases. The reduction in Voscresults in a lowering in the voltage at terminal 5 which causes afurther decrease in the output voltage of Op-amp lC2 b.

This has the effect of drawing more current through D1 which reduces thecurrent flowing into the base of TR1 and therefore further reducingvoltage Vosc and hence P out, ie. positive feedback occurs. In thismanner, Vosc reduces and P out. falls correspondingly.

Although the circuitry of FIG. 5 attempts to counter the change in thevalue of Vosc by increasing the output of IC2 a, since D1 is conductingthe increase in the current is not all directed to the base of TR1 andthe net effect is a reduction in the base current.

Since output power P out decreases, voltage V IRO also decreases and thecircuit stabilises at a lower level of output power.

The circuit provides a system whereby the discharge power decreases asthe spray apparatus is brought close to the workpiece and thereforeavoids the problems experienced with existing spray apparatus.

If the spray apparatus is moved away from the workpiece, P out increasesand the reverse of the above described changes occur. Hence, thedischarge current and voltage characteristics of the spray apparatus asit is moved towards and away from the workpiece are identical, i.e thedischarge current and voltage characteristics are reversible.

In a further embodiment of the invention (not shown), circuit 10comprises a proportional control valve, otherwise known as an electronicpneumatic regulator or an E-P converter, which controls the volume ofpowder sprayed by the gun in response to a decrease or increase in thedischarge voltage. The powder is caused to flow to the discharge nozzleof the spray apparatus by compressed air and said proportional controlvalve may control the pressure of the compressed air. Hence, as the gunapproaches the workpiece, the proportional control valve automaticallyreduces the volume of powder being charged. Since V osc is proportionalto the discharge voltage and consequently the discharge power, theproportional control valve conveniently acts upon a change in V osc.

With reference to FIG. 7, a further embodiment of the invention will nowbe described. The electrostatic powder spray coating apparatus comprisesa hopper 20 containing powder paint, a volumetric screw feeder device22, a compressed air venturi 24, a spray gun and control circuitry 26e.g. see circuitry described above with reference to FIG. 4, and controlmeans 28.

The compressed air venturi has the ability to suck powder from the screwfeeder device 22 via tube 30 and blow the powder to the spray gunthrough hose 32. The volume of powder delivered to venturi 24 isdependent upon the rotational speed of the screw which is controlled bycontrol means 28. The control means 28 monitors the discharge voltage ofthe gun and controls the speed of the screw accordingly.

The operation of the apparatus will now be described with, initially,the gun operating at its usual distance from a workpiece. With referenceto FIGS. 1 to 3 it will be seen that as the gun approaches theworkpiece, the discharge voltage decreases. The fall in dischargevoltage is detected by the control means which acts to reduce the speedof rotation of the screw thereby reducing the volume of powder deliveredto the venturi and consequently the volume of powder which is sprayed.As the gun continues to approach the workpiece the discharge voltagewill continue to fall and so, accordingly, will the volume of powderdelivered to the venturi. The reverse action occurs when the gun ispulled away from the workpiece, i.e. the discharge voltage increases andthe volume of powder delivered also increases.

The above described embodiment simply describes one method ofcontrolling the flow of powder to the electrostatic charging means but,in practice, the flow can be controlled in a number of ways. Forexample, the pressure and flow rate of the compressed air can becontrolled so as to vary automatically the volume of powder flowing tothe electrostatic charging means when the discharge voltage varies.

With reference to FIG. 8, a further embodiment of the present inventionwill now be described. An internal charging gun which forms part of anelectrostatic powder spray coating apparatus is indicated generally bynumeral 40. The internal charging gun comprises a duct 42 in which acorona discharge needle electrode 44 is located, an earth ring electrode46 which surrounds the tip of the corona needle electrode and controlcircuit 48, illustrated in FIGS. 4, 5 and 6 and described above.

In use, gas borne powder is passed through duct 42 and electrostaticallycharged by the operation of the corona needle electrode at apredetermined charge. During the operation of the gun, it is usual forconditions to change. A typical example of this is an increase in thedensity of the powder flowing through the duct. In such circumstances,the resistance between the earth ring electrode and the corona needleelectrode will increase and, in the manner described above, thedischarge current and discharge voltage will increase to compensatethereby maintaining optimum charging energy. It will be appreciated thatan increase in resistance between the earth ring electrode and thecorona needle electrode results in the same response by circuit 48 aswhen, in the above described embodiments, the gun moves away from aworkpiece.

What is claimed is:
 1. A method for controlling operation of a powderspray coating apparatus in order to improve powder coating, comprisingcontrolling output power charging a powder by progressively reducingboth a discharge current and a discharge voltage as the spray coatingapparatus approaches a workpiece wherein said reduction in the dischargecurrent and discharge voltage commences at a distance of separationbetween the spray coating apparatus and workpiece which is at least 100mm to provide substantially reproducible coating results duringoperation of the powder spray coating apparatus.
 2. A method as claimedin claim 1 wherein the reduction in the discharge current and dischargevoltage is reversible as the spray coating apparatus moves away from theworkpiece.
 3. A method as claimed in claim 1 wherein the powder spraycoating apparatus comprises means for automatically reducing saiddischarge current and discharge voltage as the spray coating apparatusapproaches the workpiece.
 4. A method as claimed in claim 3 wherein saidmeans for automatically reducing the discharge current and dischargevoltage also automatically increases the discharge current and dischargevoltage as the spray apparatus moves away from the workpiece.
 5. Amethod as claimed in claim 4 wherein the discharge current is limited soas not to exceed a maximum valve.
 6. A method as claimed in claim 5wherein said maximum limit is 50 μA.
 7. A method as claimed in claim 5wherein the upper limit is 30 μA.
 8. A method as claimed in claim 5,wherein said means for reducing the discharge current and dischargevoltage comes into operation when the discharge current reaches saidmaximum value.
 9. A method as claimed in claim 5, wherein said means forincreasing the discharge current and discharge voltage when the spraycoating apparatus moves away from the workpiece ceases its operationwhen said discharge current reaches said maximum value.
 10. A method asclaimed in claim 1 wherein the powder spray coating apparatus furthercomprises means for reducing a volume of powder dispensed by the spraycoating apparatus as the spray coating apparatus approaches theworkpiece.
 11. A method as claimed in claim 10 wherein a reduction inthe volume of powder is in response to a decrease in the output power.12. A method as claimed in claim 1, wherein a volume of powder sprayedis controlled by a change of discharge current or discharge voltage. 13.An electrostatic powder spray coating apparatus comprising anelectrostatic charging means and means to control a volume of paintpowder sprayed in response to a change in discharge current or dischargevoltage of the electrostatic charging means.
 14. An electrostatic powderspray coating apparatus as claimed in claim 13 wherein said controlmeans reduces the volume of paint powder sprayed in response to areduction in the discharge voltage and, conversely, increases the volumein response to an increase in the discharge voltage.
 15. Anelectrostatic powder spray coating apparatus as claimed in claim 13wherein the volume of paint powder sprayed is proportional to the valueof the discharge voltage.
 16. An electrostatic powder spray coatingapparatus as claimed in claim 13, wherein compressed air is utilized toconvey the paint powder to the electrostatic charging means and thecontrol means controls the flow rate of the paint powder and thereby thevolume of paint powder flowing to the electrostatic charging means. 17.An electrostatic powder spray coating apparatus as claimed in claim 16wherein the control means controls said flow rate of paint powder bycontrolling the pressure of the air.
 18. An electrostatic powder spraycoating apparatus as claimed in claim 16 wherein the control meanscontrols the flow rate by varying the size of an orifice through whichthe air passes.
 19. An electrostatic powder spray coating apparatus asclaimed in claim 16 wherein the control means controls the volume ofpaint powder flowing to the electrostatic charging means by varying thesize of an orifice through which the paint powder flows.
 20. Anelectrostatic powder spray coating apparatus as claimed in claim 16wherein said control means comprises a regulator which controls thepressure of the air such that said pressure is proportional to thedischarge voltage.
 21. An electrostatic powder spray coating apparatusas claimed in claim 20 wherein the control means comprises aproportional control valve.